Orbital data for spacecraft
Satellites, space probes and manned systems use these orbital data or orbital elements according to their function in order to allow them to orbit around the earth or travel to other planets. The angle of inclination for an orbit is critical to the mission. The spacecraft also has to be accelerated to different speeds according to the orbit.
Inclination of an orbit
Also known as the inclination of a celestial body, this is the angle between the orbital plane and a reference plane. In the solar system, the plane of the earth’s orbit, the ecliptic, is generally selected for the latter. The orbits of the large planets and the moon only deviate from this by a few degrees. The central equatorial plane of the earth is selected as a reference for earth satellites. Geostationary satellites have an inclination of 0 degrees. The international space station flies at an angle of about 59 degrees to the equatorial plane.
Periapsis
This describes the point of closest approach by a celestial body or satellite to the object being orbited. In the case of orbits around the earth, this is referred to as the perigee, while the closest orbital point to the sun of a celestial body is known as the perihelion.
Apoapsis
This describes the point at which the satellite or celestial body is the greatest distance from the object being orbited. In the case of orbits around the earth, this is referred to as the aphelion. If a satellite describes an ideal orbit about the earth, the apogee and the perigee are at the same orbital altitude.
Orbital velocity
This is the velocity of a spacecraft relative to the celestial body which forms the centre of attraction for the orbit (earth, moon, planet). If the satellite moves in a closed path about the celestial body, i.e. a centre of gravity, the velocity of the trajectory becomes the orbital velocity. In an ideal orbit, we refer to the circular orbital velocity and this is a function of the mass and the distance from the centre of gravity. The circular orbital velocity decreases as the distance increases.
Kepler’s laws of planetary motion produce variable orbital velocities for elliptical orbits. An orbital velocity of around 7.9 km/s is necessary for a spacecraft to orbit around the earth. This is referred to as the minimum velocity.
Escape velocity
This is the velocity necessary for spacecraft to leave the earth’s gravitational field and fly to the moon or other plants. In purely mathematical terms, the circular orbital velocity must be exceeded by the factor 1.414 (root 2). In other words, a planetary probe to be launched from the earth must be accelerated to a velocity of 11.2 km/s. However, the escape speed is different for each planet and depends on the relevant size of the gravitational field. If a spacecraft is to leave the solar system, it requires an initial velocity of 16.5 km/s.
